help on engine change out

I just had a mecanic or claimrd to be put a 97 chevy illumina 3100 in my 97 pontiac whitch also had a 3100v6 car acts lihe the rev limiter is kicking ig be for i can get any speed up .tach is not working dont know exactly what rpm it happens. , Innutreal it revs alot higher than when trying to take off in gear. I fired machanic .sstill need to fix this problem any Ideas where I could start to fix this